The present invention concerns cooling fans, such as fans driven by and for use in cooling an industrial or automotive engine. More particularly the invention relates to features for improving the strength and flow characteristics of automotive cooling fans.
In most industrial and automotive engine applications, an engine-driven cooling fan is utilized to blow air across a cooling system, such as a radiator. Usually the fan is driven by a belt-drive mechanism connected to the engine crankshaft.
A typical cooling fan includes a plurality of blades mounted to a central hub plate. The hub plate can be configured to provide a rotary connection to the belt-drive mechanism, for example. The size and number of fan blades is determined by the cooling requirements for the particular application. For instance, a small automotive fan may only require four blades having a diameter of 18 inches. In larger applications, a greater number of blades and a greater fan diameter may be required. In one typical heavy-duty automotive application, nine blades are included having an outer diameter of 704 mm.
In addition to the number and diameter of blades, the cooling. capacity of a particular fan is governed by the airflow volume and static efficiency that can be generated at an operating speed. Airflow volume and efficiency are dependent upon the particular blade geometry, such as blade area and blade curvature, as well as the rotational speed of the fan. Larger fan blades usually lead to greater airflow rates. Moreover, curved blades are generally more efficient than flat blades.
As the cooling fan airflow capacity increases, the loads experienced by the fan, and particularly by the blades, also increase. Increased airflow through the fan can lead to higher bending moments acting on the blades, and ultimately to increased bending stresses between blade sections. Perhaps most significantly, the higher fan speeds and flow rates can increase the stress experienced by each fan blade.
These problems become particularly acute for one-piece molded cooling fans. In order to reduce weight, most new industrial and automotive cooling systems employ fans formed of a high-strength moldable polymer material. Typically, this polymer material is injection molded about the hub plate, which is usually metallic. Weight and cost considerations frequently drive the design of such molded cooling fans, most specifically to reduce the amount of material contained within the fan. In addition, the fan configuration is typically constrained by the desire to produce the fan using only two mold halves, without the need for movable inserts.
Thus, a constant engineering tension exists between fans designed for weight and cost reduction and those designed for strength and airflow capacity. As the desire for high speed, high flow, lightweight fans increases, the design requirements for these fans become much more strenuous. The present invention provides for one solution to these apparently opposing design forces.
The present invention concerns a molded cooling fan having a plurality of blades integrated with a molded ring about a central hub plate. The plate is preferably metallic and provides means for connecting the fan to a source of rotary power. The fan can be formed using conventional molding techniques, such as injection molding. Moreover, the fan can be formed of conventional moldable materials, such as a high-strength polymer.
In one feature of the invention, the molded components of the fan have a substantially uniform thickness throughout. In other words, the molded ring and blades have substantially the same thickness. The exception to this uniformity is adjacent the blade roots, where the blade thickness is increased for strength purposes. Moreover, this uniform thickness is less than is found in the typical prior art fan. In one specific embodiment, the nominal thickness is about 3.0 mm.
In order to maintain the strength characteristics of the fan, another feature of the invention contemplates the addition of helical gussets at the molded ring on the inlet side of the fan. These gussets are in the form of a thin-walled angled fin, having its greatest height at blade root adjacent the trailing edge of each blade, and decreasing in height to the inner diameter of the molded ring. In order to prevent any disruption of the airflow across the front side of the blades, the gussets are curved and arranged in a helical pattern about the circumference of the molded ring. The gussets define airflow channels between each other, and are curved to substantially follow the effective airflow path through these channels. In certain embodiments, the airflow channels are further defined by support webs defined between the root of each blade and the molded ring.
In certain embodiments, a strengthening feature is added to the back or outlet side of the fan. In these embodiments, a number of radial ribs are integrally formed with the molded ring. A rib preferably starts at the junction of the trailing edge of each blade with the molded ring and continues to the inner diameter of the ring. The rib further has the same uniform thickness as the remainder of the molded components of the fan. A circumferential support web can be formed between the rib and the outer diameter of the molded ring. The rib and support web can combine to provide additional strength at the blade root, particularly for high pitch blades.
In another aspect of the invention, the radial ribs provide a feature to enhance the stackability of the inventive fan. More specifically, the top of the radial rib defines an inset stacking surface. This stacking surface engages a contact surface on the inlet side of the fan. The inset aspect of the stacking surface allows adjacent fans to nest within each other. The depth of the inset stacking surface determines the degree of overlap of the adjacent fans, and ultimately the reduction in stack height for a quantity of fans.
In order to accommodate the helical gussets in certain fan embodiments, the radial ribs define a clearance region that is cut out at the location of the gusset. Finally, each rib can then include a radially angled strengthening web between the clearance region and the molded ring.
The thin-walled blade construction of the present invention can create blade strength problems under maximum operating conditions. As the fan rotates, the blades are subject to inertial loads that tend to de-pitch the blades and, more critically, to generate significant stresses at the blade root and along blade sections. The present invention contemplates a blade design that addresses these problems. In one aspect of the design, the blades have an elliptical or a parabolic camber line defining the curvature from the leading edge to the trailing edge. The elliptical or parabolic camber line is calculated based on such parameters as the inlet angle at the leading edge and the outlet angle at the trailing edge. Moreover, the blade is configured so that the maximum curvature of the camber line occurs adjacent the trailing edge.
In another aspect of the invention, the blade stacking line is configured so that the centers of gravity of blade sections along its radial length are positioned to greatly reduce or eliminate bending stresses under normal operating conditions. In prior blade designs, the center of gravity at each blade section is aligned along the length of the blade under static, or non-loaded, conditions. As the fan spins up to speed, the aerodynamic loads bend the blades due to the pressure differential across the fan inlet and outlet, causing the centers of gravity to fall out of alignment. As a result, a mean bending stress is generated along the blade length that is a function of the resulting moment occurring along the blade. The maximum stress experienced by each blade is the superposition of a cyclic or alternating operating stress on the total mean stress (i.e., a combination of bending and tensile stress). In accordance with the present invention, the blade centers of gravity fall into a predetermined stacking arrangement under the normal operating loads. This feature effectively eliminates the mean bending stress, and ultimately greatly reduces the maximum total stress value.
It is one important object of the present invention to provide a molded cooling fan having reduced material requirements, while still maintaining adequate strength characteristics. Another object is accomplished by providing design features that can be readily manufactured in conventional molding processes.
One benefit of the cooling fan according to the present invention is that it easily accounts for the effects on the fan blades running at a maximum operational speed. A further benefit is that certain features of the invention provide strength where it is needed with a minimum of added material.